In recent years, reduction of amount of waste processing solutions is strongly desired in the field of medical diagnosis from the standpoints of environmental protection and space savings. Therefore, techniques relating to photothermographic materials for use in photographic-art processes and medical diagnosis are required which enables efficient exposure by a laser image setter or laser imager and formation of a clear black image having high resolution and sharpness. Such photothermographic materials can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
The same is demanded in the field of ordinary image-forming materials. However, photo-images for medical use, in particular, require high image quality excellent in sharpness and graininess as they need very fine images. In addition, for easy diagnosis, cold monochromatic images are preferred. At present, various types of hard copy systems using pigment and dye, for example, ink jet printers and electrophotographic systems are available as ordinary imaging systems. However, no satisfactory image-forming system is available for medical use.
Meanwhile, methods for forming an image by heat development are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Klosterboer, “Thermally Processed Silver Systems”, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworthand A. Shepp, Chapter 9, p. 279, (1989). Among these, the photothermographic material generally contains a photocatalyst (e.g., silver halide) in a catalytically active amount, a reducing agent, a reducible silver salt (e.g., organic acid silver salt), and optionally a toning agent for controlling silver color tone, which are usually dispersed in an organic binder matrix. In this photothermographic material when the material is heated at a high temperature (e.g., 80° C. or higher) after light exposure, black silver images are produced through an oxidation-reduction reaction between the silver halide or reducible silver salt (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image of the silver halide generated upon exposure. Therefore, the monochromatic silver images are formed in exposed areas of the materials (U.S. Pat. No. 2,910,377, Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) 43-4924 etc.). These thermal image-forming systems utilizing silver salts of organic acids can achieve image quality and color tone required for images for medical use.
The silver source used in these systems is generally a silver salt of an organic acid, and various methods for producing it have been known. For example, there can be mentioned the method of preparing a silver salt of an organic acid under coexistence of water and a hardly water-soluble solvent as disclosed in Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 49-93310, JP-A-49-94619 and JP-A-53-68702, the method of preparing a silver salt of an organic acid in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117 and JP-A-54-46709, the method of preparing a silver salt of an organic acid in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458 and so forth. Basically, the preparation is carried out by heating an organic acid to a temperature higher than its melting point to melt it in water, adding sodium hydroxide or an alkali metal salt with vigorous stirring, and then adding a solution containing silver ions in order to convert the alkali soap into silver soap.
Such alkali soap forms micelles in an aqueous solution, and gives a solution of whitely turbid appearance. The reaction from such a micelle state to the silver soap often suffers from problems concerning production stability. Therefore, as a method for obtaining the alkali soap as a uniform solution, a method of using a mixed solution of water and alcohol as the solvent is disclosed in JP-A-55-40607.
Further, since alkali soap presents alkalinity, the silver soap will be prepared under a high pH condition in the above case. However, addition of a solution containing silver ions into an alkaline solution produces silver oxide as a by-product. Further, it also generates unintended silver nuclei produced by a trace amount of reducing contaminants, which are unavoidable in view of production process and exhibit high reducing property due to the high pH. Such by-products are extremely disadvantageous from the viewpoint that they degrade performance of photothermographic materials, in particular, they cause undesired fog. In this respect, the aforementioned problems are not solved even in the method disclosed in JP-A-55-40607, which aims at obtaining a uniform solution in order to suppress the generation of the by-products.
Further, JP-A-9-127643 discloses a method for producing a silver salt by simultaneous addition of measured amounts of an alkali metal salt solution and a silver nitrate solution, and refers to simultaneous addition of a solution of sodium behenate in a mixture of water and isopropyl alcohol and a solution of silver nitrate. This method can at least shift the pH of the reaction from the high pH region to a neutral region, and thus it is a preferred method for reducing the generation amount of silver oxide. However, isopropyl alcohol shows weak reducing property, and this makes the method insufficient as a method for completely solving the problem of fog.
Further, JP-A-9-127643 discloses a method for producing a silver salt by simultaneous addition of measured amounts of an alkali metal salt solution and a silver nitrate solution, and refers to simultaneous addition of a solution of sodium behenate in a mixture of water and isopropyl alcohol and a solution of silver nitrate. This method can at least shift the pH of the reaction from the high pH region to a neutral region, and thus it is a preferred method for reducing the generation amount of silver oxide. However, isopropyl alcohol shows weak reducing property, and this makes the method insufficient as a method for completely solving the problem of fog.
Moreover, the silver behenate grains formed by this method are two-dimensionally and anisotropically grown acicular grains having a size of 0.04 μm to 0.05 μm, and no description is found concerning control of the grain size or grain morphology.
JP-A-11-349325 disclose a method for forming scaly grains, which show morphology different from that of the conventional acicular grains, by using a low temperature in a reaction field for the simultaneous addition of measured amounts of alkali metal salt solution and solution containing silver ions. In this method, scaly grains can be obtained in a low temperature region, and acicular grains can be obtained in a high temperature region by controlling the temperature of the reaction field. However, this method still cannot provide such high degree of freedom that the grain morphology and grain size can be independently controlled.
In order to obtain a uniform dispersion practically usable as a coating solution containing a silver salt of an organic acid, it is necessary to obtain a state that the silver salt of an organic acid is finely dispersed in a solvent without aggregation. For this reason, it is necessary to develop a method for dispersing the silver salt of an organic acid as fine grains. Usually used is a method comprising separating the formed hydrophobic grains of silver salt of an organic acid as solid by filtration, mixing a dispersing agent with the solid, and dispersing the mixture again, as described in Imaging Processes and Materials, supra.
As the method for dispersing a silver salt of an organic acid as fine grains, the method of mechanically dispersing it in the presence of a dispersing aid by means of known pulverization mean (e.g., high-speedmixer, homogenizer, high-speed impact mill, Banbary mixer, homomixer, kneader, ball mill, vibrating ball mill, planetary ball mill, attriter, sand mill, bead mill, colloid mill, jet mill, roller mill, trone mill and high-speed stone mill). However, this method not only produces only a coating solution containing a lot of aggregated particles, i.e., a coating solution that gives bad coated surface quality, but also suffers from a problem that, because the method highly possibly grinds primary grains of a silver salt of an organic salt originally crystallized as a hardly wafer-soluble salt without any selectivity, silver nuclei are formed at crystal cleavage surfaces and causes increase of fog.
Then, several methods have been proposed, wherein the primary grains obtained during the reaction of a solution of alkali metal salt and a solution containing silver ions are utilized as they are, not separating the silver salt of an organic acid as solid and finely dispersing it.
For example, JP-A-8-234358 discloses a method of adding silver nitrate to an aqueous dispersion in which fine grains of an alkali salt of an organic acid are dispersed, and desalting the obtained dispersion of a silver salt of an organic acid by ultrafiltration. The above reference further refers to enhancement of the dispersion stability by carrying out the ultrafiltration for a dispersion preliminarily containing water-soluble protective colloids such as polyvinyl alcohol and gelatin.
However, the shape of the silver salt of an organic acid obtained by this method is limited to an acicular shape, and in addition, it is difficult to control the grain size in this method. Therefore, it is still insufficient for stably obtaining performance of low fog, high blackening concentration and low haze, which are desired for photothermographic materials.
Further, JP-A-9-127643 discloses a method of directly desalting a dispersion of a silver salt of an organic acid obtained by simultaneous addition of measured amounts of a solution of an alkali metal salt and a silver nitrate solution by means of dialysis or ultrafiltration. By this method, at least the primary grains obtained during the crystallization of the silver salt of an organic acid can be introduced into a photosensitive layer as they are without degrading the grains. However, problems concerning aggregation of grains under a high salt concentration circumstance, increase of viscosity upon concentration of the dispersion and so forth are not solved, and thus this method is still insufficient as practical means for obtaining a uniform dispersion.
Furthermore, JP-A-9-127643 discloses a method using a dispersing agent together, like JP-A-8-234358, but it does not refer to the kind of preferred dispersing agents. This method is not a method providing superior dispersion stability, in which the grain morphology and the grain size are controlled at a high salt concentration during the generation of grains of silver salt of an organic acid in the presence of an organic solvent such as isopropyl alcohol.
In order to obtain monodispersed grains of a silver salt of an organic acid as fine grains, vigorous stirring is required during the addition of an alkali metal salt solution and a solution containing silver ions. In particular, the solution of an organic acid alkali metal salt dissolved at a high temperature suffers from temperature decrease and shows precipitation upon addition thereof, and therefore large grains may grow if dilution rate or fluidization is slow or weak. However, when they are added to a tank in which a gas/liquid interface is present, and the stirring speed is increased, entrainment of air is caused. The grains of silver salt of organic acid are highly hydrophobic, and therefore not only the grains are adsorbed on the surfaces of the entrained bubbles to stabilize the bubbles and prevent breakage of them, but also the adjacent grains on the bubbles cause aggregation. The liquid containing air entrained in such a manner becomes a highly viscous whipped cream-like liquid, and disturbs uniform reaction.
Further, when such a liquid containing bubbles and aggregated grains is subjected to ultrafiltration operation, it suffers from problems of reduction of filtration speed due to adhesion to a membrane surface, increase of filtration pressure due to increased viscosity and so forth. Furthermore, when a hollow-fiber type membrane filter is used, it suffers from problems of blocking of pipelines due to progress of aggregation of grains and so forth. These problems constitute obstacles to the application of the methods to practical production facilities.
As described above, any method that enables independent control of the grain size and the grain morphology and stably produces monodispersed silver salt of an organic acid providing low fog has not been found yet. That is, any method has not been found yet, in which organic acid silver salt grains are formed by adding and vigorously stirring a solution containing organic acid alkali metal salt and a solution containing silver ions without causing entrainment of air or aggregation of grains, and the obtained primary grains are utilized as they are without preliminarily separating the produced organic acid silver salt as solid content.
In view of these problems of the prior art, an object of the present invention is to provide an efficient method for producing grains of silver salt of an organic acid, which enables purification of the organic acid silver salt grains produced by mixing an alkali metal salt solution and a solution containing silver ions without separating the organic acid silver salt grains as a solid content. Another object of the present invention is to provide a method and apparatus for preparing grains of silver salt of an organic acid that require low facility cost and small facility space and show high productivity.